There is growing interest in high-quality digital projection systems that display images that can match or surpass the quality of film, especially in large venues. The most promising of these digital projection solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators (SLMs). The first type of spatial light modulator is the Digital Light Processor (DLP) a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DLPs have been successfully employed in digital projection systems. DLP devices are described in a number of patents, for example see U.S. Pat. No. 4,441,791; U.S. Pat. No. 5,535,047 and U.S. Pat. No. 5,600,383 (all to Hornbeck).
FIG. 1 shows a simplified block diagram of a projector apparatus 10 that uses DLP spatial light modulators. A light source 12 provides polychromatic unpolarized light into a prism assembly 14, such as a Philips prism, for example. Prism assembly 14 splits the polychromatic light into red, green, and blue component wavelength bands and directs each band onto corresponding spatial light modulators 20r, 20g and 20b. Prism assembly 14 then recombines the modulated light from the spatial light modulators 20r, 20g and 20b and provides this unpolarized light to a projection lens 29 for projection onto a display screen or other suitable surface.
DLP-based projectors demonstrate the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema. However, there are inherent resolution limitations, with existing devices typically providing no more than 2148×1080 pixels. In addition, high component and system costs have limited the suitability of DLP designs for higher-quality digital cinema projection. Moreover, the cost, size, weight and complexity of the Philips prism or other suitable combining prisms are significant constraints. In addition, the need for a relatively fast projection lens with a long working distance, due to brightness requirements, negatively impacts the acceptability and usability of these devices.
The second type of spatial light modulator used for digital projection is the Liquid Crystal Device (LCD). LCD spatial light modulators form an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs have some advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size, favorable device yields and the ability to fabricate higher resolution devices, for example the 4096×2160 resolution devices available from Sony and JVC Corporations. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 by Shimomura et al. Liquid Crystal On Silicon (LCOS) devices are thought to be particularly promising for large-scale image projection. However, with LCD components it can be difficult to maintain the high quality demands of digital cinema, particularly with regard to color and contrast, since the high thermal load of high brightness projection affects the polarization qualities of these devices.
A continuing problem with illumination efficiency relates to etendue or, similarly, to the Lagrange invariant. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image can be. Numerically, etendue is proportional to the product of two factors, namely the image area and the numerical aperture. In terms of the simplified optical system represented in FIG. 2 having light source 12, optics 18, and a spatial light modulator 20, etendue is a product of light source area A1 and its output angle θ1, and, in a well-matched optical system, this is equal to the product of the modulator area A2 and its acceptance angle θ2. For increased brightness, it is desirable to provide as much light as possible from the area of light source 12. As a general principle, the optical design is advantaged when the etendue at the light source is most closely matched to the etendue at the spatial light modulator.
Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue. In order to utilize an increased etendue on the illumination side, the etendue at the spatial light modulator must be greater than or equal to that of the illumination source. Larger image sizes, however, typically result in a more costly system. This is especially true of LCOS and DLP components, where the silicon substrate and defect potential increase with size. As a general rule, increased etendue results in a more complex and costly optical design. Using a conventional approach such as that outlined in U.S. Pat. No. 5,907,437 by Sprotbery et al., for example, lens components in the optical system must be designed for large etendue. The source image area for the light that must be converged through system optics is the sum of the combined areas of the spatial light modulators in red, green, and blue light paths; notably, this is three times the area of the final multicolor image formed. That is, for the configuration disclosed in such a conventional approach, optical components handle a sizable image area, therefore a high etendue, since red, green, and blue color paths are separate and must be optically converged. Moreover, although a configuration such as that disclosed in the Sprotbery et al. patent handles light from three times the area of the final multicolor image formed, this configuration does not afford any benefit of increased brightness, since each color path contains only one-third of the total light level.
Efficiency improves when the etendue of the light source is well-matched to the etendue of the spatial light modulator. Poorly matched etendue means that the optical system is either light-starved, unable to provide sufficient light to the spatial light modulators, or inefficient, effectively discarding a substantial portion of the light that is generated for modulation.
The goal of providing sufficient brightness for digital cinema applications at an acceptable system cost has eluded designers of both LCD and DLP systems. LCD-based systems have been compromised by the requirement for polarized light, reducing efficiency and increasing etendue, even where polarization recovery techniques are used. DLP device designs, not requiring polarized light, have proven to be somewhat more efficient, but still require expensive, short lived lamps and costly optical engines, making them too expensive to compete against conventional cinema projection equipment.
In order to compete with conventional high-end film-based projection systems and provide what has been termed electronic or digital cinema, digital projectors must be capable of achieving comparable cinema brightness levels to this earlier equipment. As some idea of scale, the typical theatre requires on the order of 10,000 lumens projected onto screen sizes on the order of 40 feet in diagonal. The range of screens requires anywhere from 5,000 lumens to upwards of 40,000 lumens. In addition to this demanding brightness requirement, these projectors must also deliver high resolution (2048×1080 pixels) and provide around 2000:1 contrast and a wide color gamut.
Some digital cinema projector designs have proved to be capable of this level of performance. However, high equipment cost and operational costs have been obstacles. Projection apparatus that meet these requirements typically cost in excess of $50,000 each and utilize high wattage Xenon arc lamps that need replacement at intervals between 500-2000 hours, with typical replacement cost often exceeding $1000. The large etendue of the Xenon lamp has considerable impact on cost and complexity, since it necessitates relatively fast optics to collect and project light from these sources.
One drawback common to both DLP and LCOS LCD spatial light modulators has been their limited ability to use laser light sources. Although they are advantaged over other types of light sources with regard to relative spectral purity and potentially high brightness levels, laser light sources require different approaches in order to use these advantages effectively. Conventional methods and devices for conditioning, redirecting, and combining light from color sources, used with earlier digital projector designs, can constrain how well laser light sources are used.
Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability, but, until recently, have not been able to deliver visible light at sufficient levels and at costs acceptable for digital cinema. In a more recent development, laser arrays have been commercialized and show some promise as potential light sources. However, brightness itself is not yet high enough; the combined light from as many as 9 individual arrays is needed in order to provide the necessary brightness for each color.
Laser arrays of particular interest for projection applications include various types of VCSEL (Vertical Cavity Surface-Emitting Laser) arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Novalux, Sunnyvale, Calif. However, conventional solutions using these devices have been prone to a number of problems. One limitation relates to device yields. Due largely to heat and packaging problems for critical components, the commercialized VECSEL array is extended in length, but limited in height; typically, a VECSEL array has only two rows of emitting components. The use of more than two rows tends to increase yield difficulties dramatically. In addition, conventional VECSEL designs are prone to difficulties with power connection and heat sinking. These lasers are of high power; for example, a single row laser device, frequency doubled into a two-row device by Novalux, produces over 3 W of usable light. Thus, there can be significant current requirements and heat load from the unused current. Lifetime and beam quality is highly dependent upon stable temperature maintenance.
Coupling of the laser sources to the projection system presents another difficulty that is not adequately addressed using conventional approaches. For example, using Novalux NESEL lasers, approximately nine 2 row by 24 laser arrays are required for each color in order to approximate the 10,000 lumen requirement of most theatres. It is desirable to separate these sources, as well as the electronic delivery and connection and the associated heat from the main thermally sensitive optical system to allow optimal performance of the projection engine. Other laser sources are possible, such as conventional edge emitting laser diodes. However, these are more difficult to package in array form and traditionally have a shorter lifetime at higher brightness levels.
The use of laser sources in general presents its own set of imaging artifacts. The primary artifacts of concern are that of laser speckle and illumination uniformity. Speckle is a fine scale spatially varying intensity fluctuation that is caused by random roughness of optical surfaces on the order of a wavelength of light. The increased coherence of lasers introduces a significant effect in projection systems where the roughness creates randomly phased sub-sources interfering together. This random intensity fluctuation lowers the effective MTF of an image, especially at the higher frequencies essentially producing a “shimmer effect” in detail, but also creating an intensity sharpness that is really artificial. The phenomenon of speckle has been studied in detail by many researchers and a comprehensive summary of knowledge has been published by Joseph Goodman, Speckle Phenomena in Optics: Theory and Application, (Roberts & Company Publishers, Greenwood Village, Colo., 2007). Goodman indicates that full-frame displays should at least have the standard deviation intensity variation less than that of the least significant bit of the intensity resolution of the modulation device. For Digital Cinema applications this control at 12 bits and contrast ratios of around 2000:1 are common. Other cinema standards lean toward different criteria, indicating that speckle “should not be visible”, this can be quantitatively assumed to have the level of speckle to be equivalent to that of a white light projector on a common screen. This has been measured to be around 3% peak to valley variation in intensity.
Goodman has characterized some common approaches to reducing speckle in display applications:                1. Introduce polarization diversity;        2. Introduce a moving screen;        3. Introduce a specially designed screen that minimizes the generation of speckle;        4. For each color, broaden the spectrum of the sources or use multiple lasers at slightly different frequencies, thereby achieving wavelength diversity in the illumination;        5. For each color, use multiple independent lasers separated spatially, thereby achieving angle diversity in the illumination;        6. Overdesign the projection optics as compared with the resolution of the eye;        7. Image a changing diffuser with random phase cells onto the screen; and        8. Image a changing diffuser with deterministic or orthogonal phase codes onto the screen.Each of these approaches has some benefits as well as negative attributes. Some of these apply well for high-end digital cinema projection, while others do not. In addition, in many cases a single approach may not be effective enough to reduce the speckle below acceptable thresholds.        
Polarization diversity is not desirable in many cases, as any projector that requires polarization either to modulate the light or to create stereoscopic imaging cannot allow impure states to reach the viewer. Specially designed screens and screen shaking can be effective; however, this requires modification to the venue, which is undesirable because being able to show a quality on any screen is preferred. Similarly, if a polarization maintaining screen is desired adding additional constraints or features may be prohibitively expensive or difficult. Manufacturing processes used to make large screens are especially difficult to modify because the equipment is large and expensive.
Spectrally broadening may be desirable, however, this may be difficult to control in the laser fabrication, as many methods of creating visible solid state sources desired for display applications use frequency double crystals that control the wavelength to around 1 nm.
Multiple independent lasers are a very good approach, but this solution depends on the number of elements used to control the speckle. This does not work well over the range of light levels encountered from low-light-level to high-light-level projection systems, because a 1000 lumen projector needs to be as speckle free as a 10,000 lumen projector, even though the number of sources may be a factor of 10 times less for the 1000 lumen projector. For example, Mooradian et al, disclose an improved speckle performance in their paper “High Power Extended Vertical Cavity Surface Emitting Diode Lasers and Arrays and Their Applications” published in the proceedings of the Micro-Optics Conference, held in Tokyo, Japan on Nov. 2, 2005. In this case 30 to 40 independent (incoherent to each other) emitters reduced the speckle down to several percent. While the speckle is reduced with larger number of emitters it is not always reduced to white light levels necessitated by the stringent digital cinema requirements.
Changing diffusers can be very effective in some configurations. However, this increases the etendue of the sources by creating additional angular extent and requires a relatively expensive and controlled surface treatment on a glass surface in order to be durable for high-powered systems.
In U.S. Pat. No. 7,296,987 Mooradian et al. disclose individual and combined techniques to reduce laser speckle similar to those described by Goodman. First, they describe increasing the number of lasers that are substantially incoherent with respect to each other. Second, they also teach that spectral broadening of the lasers may be used. (This technique is also described in U.S. Pat. No. 6,975,294 by Manni et al.) Third, they disclose that individual lasers in an array may be designed to operate with multiple frequency, phase, and directional (angular) distributions. Finally, they describe using an optical element to scramble the direction, phase and polarization information. As described earlier, increasing the number of lasers is effective at reducing speckle, however is incomplete. The additional methods described are difficult to implement, expensive or undesirable optically.
U.S. Pat. No. 7,244,028 by Govorkov et al. describes the use of at least one laser delivered to a scanning means that increases the laser beam divergence temporally into a lens that delivers the light to a beam homogenizer that illuminates a spatial light modulator, this reduces the laser speckle to acceptable levels when combined with a screen that has at least one feature to further reduce speckle. Temporally varying the laser beam divergence is generally a good means of reducing speckle, however it too requires the modification of the screen for complete speckle reduction. This is undesirable for general projection purposes.
U.S. Pat. No. 7,116,017 by Ji et al. describes a specific device consisting of a vibrating mirror in the light path between the laser and the screen. This alone will not reduce speckle to acceptable levels. U.S. Pat. No. 6,445,487 by Roddy et al. describes methods that use frequency modulation of the lasers in conjunction with a device to deviate the beam angularly in time. This method requires laser modulation that may not be practical or possible for all laser sources. Similarly the application focuses on using an acousto-optic modulator for angular deviation. These devices are very expensive and can only handle certain laser types and sizes.
While laser speckle has been the artifact given the most attention in laser projection systems, other artifacts resulting from the coherent nature of lasers can become problems. Speckle is an artifact caused by the small features on the order of a wavelength on the optical surfaces in the system, each creating a new relative source. Generalized interference can also occur from fill surface wavefront interference or more macro-level structures in the system. One prime example of this is interference created from uniformizing optics. Uniformizing optics are typically comprised of optical elements that, either in combination or independently, overlap sources in an angular or spatial manner. With incoherent light this would provide a mixing of initially non-uniform intensity to create a uniform, averaged intensity output. Devices such as integrating bars and spaced lenslet arrays are examples of techniques that are common in the art. However, utilizing these devices to uniformize one or more coherent light sources creates many opportunities for the overlapping of phased wavefronts, thus resulting in multiple interference artifacts. Therefore, both lenslet arrays and integrating bars are plagued with interference fringes associated with the pattern of the overlapping angular and spatial wavefronts. This structure is unacceptable for imaging a uniform data set from the spatial light modulator.
Thus, it can be seen that the challenge of providing a color laser projection system having cinema quality in uniformity and brightness has not been met.
There is a need for a coherent illumination solution that enables uniform interference free imaging for high-end projection systems.